The field-effect transistor (FET) has been used as biochemical sensors since the first introduction of the ion-selective field effect transistors (ISFET) in 1970, See, for example, P. Bergveld, IEEE Trans. Biomed Eng. 17, 70, (1970), The basic structure of an ISFET comprises an insulator-semiconductor junction FET with a non-metallized gate serving as the sensor where the analyte solution is sensed. ISFET devices have been used popularly as a sensitive pH sensor and various biochemical sensors. See, for example, P. Begveld, Sens. Actuators B 88, 1-20, (2003).
Since then, the development of more sensitive versions of the ISFET has been geared towards the selection of the inorganic material used for the non metallized gate such as Si3N4 and Ta2O5. See, for example, M. Asahi, and T. Matsuo, Suppl. Jpn. Soy, Appl. Phys. 44, 339, (1975); P. Gimmel, K. D. Schierbaum, Gopel, H. H. Van den Vlekkert, and N. F. deRooy, Sens. Actuators B 1, 345, (1990).
More recently the biosensing ISFET mechanism has been applied to Poly-Si thin film transistors (TFTs) and GaN/AlGaN high electron mobility transistors (HEMTs) for detection of DNA, penicillin, and cellular potentials. See, for example, P. Estela, A. G. Stewart, F. Yan, and P. Migliorato, Electrochimca Acta 50, 4995-5000, (2005); J. Yu, S. K. Jha, L. Xiao, Q. Liu, P. Wang, C. Surya, and M. Yang, Biosens. Bioelectron. 23, 513-519, (2007). However, the entire gate of this type of devices serves as the sensing area which contains the analyte solution. The biasing of the ISFET can be invasive as it is done by using a reference electrode dipped directly into the analyte on the gate oxide. This configuration may also disturb the solution being detected, especially if the bias is too high.
Another class of FET-type of biosensors is based on organic field-effect transistors (OFETs), which are usually fabricated at low temperatures on various substrates including glass and flexible polymers. The general structure of an OFET includes a back-gate MOSFET with the conducting channel, made of organic semiconductors. The analyte can be in any of three locations on the exposed organic channel as in the case of gas sensors, or between the insulator and gate layers as in the case of ion sensitive sensors (pH sensors), or the analyte can act as the insulator itself between the organic semiconductor and the gate, as in the case of the electrochemical sensors. See, for example, J. T. Mabeck1, and G. G. Malliaras, Anal. Bioanal. Chem. 384, 343-353, (2006). Since the first OFET was introduced, research efforts in OFET biochemical sensors have focused on development of various organic semiconductors to increase the device's sensitivity and selectivity in detecting wide range of chemicals including gases, enzymes, and DNA. See, for example, F. Ebisawa, T. Kurokawa. S. Nara, J. Appl. Phys. 54, 3255-3259, (1983); L. Torsi, A. Dodabalapur, L. Sabbatini, and P. G. Zambonin, Sens. Actuators B 67, 312-316, (2000); J. Liu, M. Agrawal, and K Varahramyan, Sens. Actuators B 135, 195 199, (2008); Q. Zhang, and V. Subramanian, Biosens. Bioelectron. 22, 3182-3187, (2007). The OFET has the advantage of being easily controlled through biasing due to the back-gate configuration. However, the level of bias voltage required to operate OFETs is generally high, which could cause unwanted electrochemical reactions during, testing. Furthermore, the OFETs have low mobility and on-off ratios under the normal voltage biasing, The low output current levels lead to small signal to noise ratios, making the sensors susceptible to noise.
Currently, nanowire-based FET sensors are demonstrated with high sensitivity reaching the order of fM. See, for example, K. S. Chang, C. C. Chen. J. T. Sheu, and Y.-K. Li, Sens, Actuators B 138, 148-153, (2009). However, these prototypes of sensors generally involve a complex fabrication process as they are constructed individually by manipulating and aligning a single strand of semiconducting nanowire such as TiO2 or Si as the FET channel between the source and drain patterns. It is difficult to achieve repeatability and manufacturability in fabrication and integration of these devices for larger sensor arrays.
ZnO is emerging as a wide handgap semiconductor oxide with multifunctional properties that makes it an attractive sensor material. ZnO is highly sensitive to various molecules including CH4, CO, H2O, H2, NH3, trimethylamine, ethanol and NO2. See, for example, V. I. Anisimkin, M. Penza, A. Valentini, F. Quaranta, and L. Vasanelli, Sens Actuators B 23, 197. (1995); T.-J. Hsueh, S-J. Chang, C-L. Hsu, Y-R. Lin, I-C. Chen, Appl. Phys. Left, 91, 053111 (2007). ZnO and its nanostructures are compatible with intracellular material and ZnO-based sensors have been demonstrated for detection of biochemicals such as enzymes, antibodies, DNA immobilization and hybridization. See, for example, S. M. Al-Hilli, R. T. Al-Mofarji, and M. Willander, Appl. Phys. Lett. 89, 17, 173119 (2006); A. Wei, X. W. Sun, J. X. Wang, Y. Lei, X, P. Cai, C. M. Li, Z. L. Dong, W. Huang, Appl. Phys. Lett, 89, 12, 123902. (2006); P. I. Reyes, Z. Zhang, H. Chen, Z. Duan, J. Zhong, G, Saraf, Y. Lu, O. Taratula, E. Galoppini, N. N. Boustany, IEEE J. Sens, 10, 2030250, (2009); Z. Zhang, N. W. Emanetoglu, Saraf, Y. Chen, P. Wu, J. Zhong, Y. Lu, J. Chen, O. Mirochnitchenko, M. Inouye, IEEE Trans. Ultrasonics, Ferroelect. Freq. Contr. 53, 4, 786-792, (2006).
Accordingly, there is an immediate need for improved sensors and sensing methods.